Part of the reason this can seem complicated is due to the different layers involved and some confusing terminology.
Most of the time, reference to 'GNU Linux console' refers to using the virtual console i.e. no GUI, no X and not an xterm. People often refer to working in the 'linux console' when they mean working at the command line shell in an xterm. To further confuse matters, there are multiple xterm emulators, so even when they speak of xterm, there are different variants.
The reason this is relevant is that the issue you are experiencing can be caused at many levels. If you are working in a real GNU linux console, then you probably need to install an appropriate keymap for the console before Emacs will see Alt+ key combinations. If on the other hand, you are actually running emacs from a shell running in an xterm emulator, then it is highly likely that the xterm emulator is not passing the Alt+ key combinations through and Emacs is simply not seeing them. things become complicated because some emulators do and some don't and they all seem to have different ways for changing the behavior.
If your using an xterm emulator which honours the traditional XTerm class setting via .Xresources/.Xdefualts, you need to add
to the configuration and either restart your session or do an xrdb --merge to add that change into the database.
Note that restart your session means restart your X session, not just the xterm you are using to run emacs.